1. Field of the Invention
This invention relates to devices which use on-chip one-time-programmable memory, including, but not limited to, secure devices in the field of pay-television.
2. Description of the Related Art
Many systems use on-chip one-time-programmable (OTP) memory which is memory that can be written to once only. For example, in the field of pay-television, a set-top-box may use on-chip OTP in scrambling circuits for storing encryption keys for system message delivery and authentication. In view of the nature of many applications, including the example above, it is important that the contents of the memory remains secret. Placing the memory on the chip itself increases the security of the memory and prevents unauthorised access to the sensitive data.
One problem with on-chip OTP is that it is expensive in both manufacturing costs and chip area. For example, a 256 byte memory may require a chip area of 1 mm2, which may be as much as 5% of the total chip area. It may be desirable to provide larger memories but the increase in chip size and manufacturing costs may become prohibitive.
One solution to this problem is to emulate on-chip memory using external flash memory where data is stored in the external flash in an encrypted form to maintain security. This solution has the disadvantage of being vulnerable to ‘replay attacks’ where old data stored in the external flash memory is replaced. Cloning may also be possible where the external flash memory is physically removed and replaced by a new memory. These disadvantages each increase the likelihood of a hacker being able to breach the overall security of the system.
We have appreciated the difficulties mentioned above. The present invention overcomes these disadvantages and provides for secure emulated on-chip OTP memory.